Having been in the law enforcement profession, there are several factors when it comes to being paid in this field. Leila's answers are spot on and given all things considers, an entry level criminal investor at the FBI for instance starts around $39K but a seasoned agent can make upwards of $100K per year. Another good point is if you start at a small city or township, your salary could be between $25K-$50 per year. First, you will need to determine if you want to be an investor with arrest powers or a background investor. Once you have narrowed your career field down, then you can search on salary ranges. I wouldn't pick a career that paid the most, pick a career path you can be passionate about.
Hope this help!
Regarding your question, it really depends. Several factors come into play: experience, location etc.
Depending on whether you're employer is a city, county, state, the federal government etc..., depending on where you work, the salary can range from anything from $30k to $80k+ a year.
Signed: Data Nerd for Goods